Network throughput between a Linux backup server and its storage array is far below expected. The server's eth1 interface is configured with an MTU of 9000, but the upstream switch is still at the default 1500. Before changing any interface settings, the administrator wants to prove that oversized packets are being dropped by the path. Which one-line command, executed from the Linux server, will most directly confirm that mismatch?
The quickest way to demonstrate an MTU mismatch is to send an ICMP echo request that is larger than the path MTU and forbid fragmentation. In Linux, the ping command with the -M do option (set the DF bit and prohibit local fragmentation) and -s 8972 (to create a 9000-byte frame, accounting for the 28-byte ICMP/IP overhead) achieves this. If the upstream switch only supports 1500-byte frames, this command will fail with a "Message too long" error or 100% packet loss, directly proving the fault.
ping -M do -s 8972 10.0.0.50 is the correct answer. It sends a large 9000-byte packet with the "Don't Fragment" bit set, which will be dropped by the switch, confirming the MTU mismatch.
ping -M want -s 64 10.0.0.50 is incorrect. It uses a small 64-byte payload that would not exceed the MTU. The want option also allows for fragmentation if needed, which would mask the issue.
ping -c 5 -f 10.0.0.50 is incorrect. This command floods the destination with default-sized (small) packets and does not test for large frame issues.
traceroute -d 10.0.0.50 is incorrect. While traceroute is a useful network diagnostic tool, in its default mode it does not test for MTU path limits.
Therefore, sending a large, non-fragmentable ping is the most direct way to confirm an MTU mismatch.
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CompTIA Linux+ XK0-006 (V8)
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